The inside of the wrist isn't hairy
Would it have worked for that segment of oddballs who wear their watch wrong way around? For those who wear it on top of their wrist, it it would require putting sensors in the band instead on the back of the watch. Also, I would have expected in order to measure blood pressure it would need to fit rather snugly, wouldn't it?
It would be nice to have the ability to monitor more stuff, but if it was easy you'd think someone would have done it before. Apple probably had it working well in controlled tests, but found it wasn't nearly so reliable when people were wearing the watches in a more typical day to day fashion.